The present invention relates to a system for connecting a low-pressure jet to a transition duct which form part of a gas turbine.
Furthermore the present invention relates to a low-pressure jet which forms part of a gas turbine.
Gas turbines are known to comprise a compressor to which ambient air is fed and then pressurised.
The pressurised air passes through a series of combustion chambers terminating in a jet in each of which an injector supplies fuel which mixes with the air to form an air-fuel mix for combustion.
The combustion gases are then sent towards the turbine which transforms the enthalpy of the gases which have been combusted in the combustion chamber into mechanical energy available to a user.
Twin-shaft gas turbines have a gas generator and a power turbine with independent shafts which rotate at different speeds.
The power rotor is made up of a shaft which at one end supports the low-pressure turbine discs and the bearing flange at the other end.
The hot gases generated in the gas generator have to be converted to power available to a user by a low-pressure turbine.
The low-pressure jets accelerate and direct the hot gases towards the rotor blades, transferring the useful power to the turbine shaft.
In order to keep the housing of a gas turbine as cool as possible, the stator jets over which the hot gases pass are not fixed directly to the stator housing but instead are arranged in sectors, each sector being made up of two or three jets, and, are insulated from the stator housing by means of dividers made of low heat conductivity material.
It should be noted here that the transition duct is the connecting component between the high-pressure blades and the low-pressure jets.
In particular, the transition duct generally consists of two concentric cylinders whose middle section may have a different shape so as to allow the desired flow of hot gases.
The hot gases have to flow in a way that minimises mechanical and heat losses. The hot gases are at very high temperatures and, therefore, the walls of the transition duct must be able to withstand these conditions and maintain their structural integrity.
Furthermore, the sealing between the transition duct and the low-pressure jets is an important variable for ensuring the performance of the motor and for maintaining the integrity of the outer housing of the gas turbine.
In order to promote understanding, of the technical problems to be solved by the invention, reference should now be made to the following state of the prior art.
Transition ducts of existing turbines may be divided into two categories: high-thickness transition ducts (which are divided into segments) and low-thickness transition ducts (which are parts at 360°).
In both solutions, the housings and the low-pressure jets can easily support the external walls.
The main problem to be solved is that of correctly supporting the internal wall which generally faces the rear of the high-pressure rotor.
Therefore, the internal wall may only be supported by an external wall using structural struts or connecting arms or protuberances which extend from the platform of the low-pressure jets.
The first solution is particularly advantageous if the axial length of the transition duct is considerable, as the internal wall cannot be easily supported at one end only.
However, the struts or connecting arms can cause mechanical losses and are not of simple design, precisely because of the high temperatures of the gases which flow through the transition ducts.
These high temperatures can cause incidents of “heat fatigue” which can cause various components to break and therefore significantly limit their useful life and their resistance to heat fatigue.
In any case the axial length of the transition duct can only be minimised by extending the axial length of the platform of the low-pressure jets so that the transition duct is included in the extended platforms of the low-pressure jets in some applications.
This solution is shown in FIG. 1 as an example of an embodiment of the prior art.
In particular FIG. 1 shows an assembly indicated generally by reference numeral 20, comprising a transition duct 21 connected to the extended platform 22 of the low-pressure jets 23. A connecting arm 24 is also shown.
The segmented internal walls can also be supported by an internal structure that is protected from hot gases, and are subject to weight loads and pressure differentials.
In the prior art, segmented transition ducts are commonly used for high-performance motors with horizontally separated housings alongside the (thin-walled) 360° transition ducts which are common in aeronautical turbines.